If the slip spline chamber is sealed to accommodate high pressure, the spline cannot be easily moved and reaction forces in the supporting end machinery become larger. To minimize these forces, among others, a felt or lip type rubber seal is used, which leaks at relatively low positive pressure. The reverse action of a conventional lubrication fitting one-way valve allows the chamber to open to atmosphere when the extension stroke produces a vacuum. If the slip spline chamber is filled with lubricant, the pump action will purge lubricant out through the low pressure seal, and will permit intake of air into the chamber through the one-way lubricant valve fitting. At some point, the pressure required to compress the air volume within the chamber to compensate for internal volume change in the chamber will equal the seal leak pressure, and lubricant leakage through the seal will cease. This equilibrium condition occurs only if rotational speed is low and slip spline movement is slow, and then both felt and lip type seals perform quite satisfactorily. However, many universal joint applications require rapid spline movement at high operating speeds, and prior art slip spline designs which are subjected to high speed and rapid slip have two major deficiencies:
(1) The static seal leak pressure, which is set at a low level to minimize forces requires to move the spline, is further diminished by the effect of centrifugal force which lifts the seal lip away from the female spline member. This reduction in seal leak pressure in turn causes two problems: PA1 (2) Rapid movement of the slip spline tends to produce high lubricant flow through the small clearances in the slip spline teeth which in turn causes high pressure in the lubricant adjacent to the seal. Since the seal leak pressure is relatively low, the lubricant does not transfer, instead it purges out the seal. The centrifugal effect forces lubricant slowly through the spline to fill the void adjacent to the seal, and the cycle repeats itself until no further lubricant remains in the chamber.
(a) The equilibrium established between seal leak pressure and internal air pressure is now altered and the slip spline pumping action purges lubricant through the seal until a new pressure balance is attained. PA2 (b) Rotation of the slip spline chamber produces pressure in the annulus volume of lubricant which exceeds the seal leak pressure and lubricant leaks through the seal.